1. Field of the Invention
The invention relates to metallized dielectric capacitors, and in particular to means for protecting the relatively thin electrical connections in such capacitors from destruction by high current surges.
2. Description of the Prior Art
Metallized dielectric capacitors are popular because their thin metallic layer structure enables a large capacitance to be produced in a relatively small volume. However, such capacitors are easily destroyed by high current surges which tend to vaporize the metallic layer where it is electrically connected to lead wires or other means for connecting the capacitor in a circuit.
Typical circuits in which such destructive current surges occur are motor-start circuits and lamp-ballast circuits where the capacitor is wired to inductive loads which produce short duration current spikes during startup. Although protective surge suppressors can be wired in series with the capacitor, this detracts from the small size advantage of the metallized dielectric capacitor and increases the complexity and cost of manufacturing the circuits.
U.S. Pat. No. 4,635,163 proposes incorporating a thermal circuit breaker device within the hollow core of a capacitor disposed in a sealed casing. The device is a fusible metal strip which fits easily within the core. This device protects the capacitor against short circuits of sufficient duration and magnitude to melt a portion of the strip, but cannot provide protection against short duration current surges such as spikes which last only a few milliseconds. Further, the fusible device is destroyed when it performs its protective function, thus necessitating replacement of the capacitor.